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Full text of "Text Book Of Mechanical Engineering"

Appendix VI.

1047

P. 204. A Worm Gear Pulley Block.—It has already
been stated that in the differential pulley block the load will not
overhaul, that is, will remain hanging when the hands are taken
off the lifting chain. Let us examine the price to be paid for this
in frictional loss. In any machine, where P and Pj are the
theoretical and practical efforts respectively, W the load, and R
the velocity ratio,

W x i   =  P x R       .      and

"*

And the frictional effort =

W

"R

If now we require that the load shall be just balanced by the
frictional resistance, we have

Wxi

And

But in that case

3
	R/R =
	PI*

W
	R 2W
	W

R*
	._
	2~W

So that, if the load is not to overcome the frictional resistance,,
the efficiency of the machine cannot be greater than 50%. In-
deed, in the Westou block it is only 30%, or 70% of the manual
energy is lost in useless resistance. The Moore block, p. 525, is
somewhat better, having an efficiency of about 40 to 45%. A
well-cut worm gear will give yet more economical results, but is
not proof against overhaul at any velocity ratio equal with that of
differential blocks. The difficulty is overcome in Herbert
Morris and Bastert's pulley block, Fig. 943, by an automatic
friction brake wheel, which sustains the load when standing, but
does not absorb any portion of the energy of lifting. The sup-
porting hook A is attached to a bracket B in which are bearings
H, F, and G for the worm wheel c and the worm E. The sling
hook Q and snatch block P are supported by a chain N M, whose
standirig end L is connected to the bracket B, while the free end
N is of any convenient length for the height of lift required; and
this chain is raised by means of the sprocket D fixed to the worm